feat: add live-qemu target with 12GB RAM for nix package headroom

Live session is a pure-RAM tmpfs overlay. Cinnamon takes ~2.5GB, nix
packages take ~4GB — 6GB QEMU was too tight. 12GB gives comfortable
headroom. Real XPS 9700 (32GB) already works fine.

- tests/launch-live-qemu.sh: dedicated QEMU launcher, RAM_MB=12288
- Makefile: 'make live-qemu' target

Makefile

@@ -15,7 +15,7 @@ PROJECT_DIR := $(CURDIR)
 OUT         := $(PROJECT_DIR)/out
 SECRETS     := $(PROJECT_DIR)/secrets.env
 
-.PHONY: all iso live test test-disk test-iso qemu shellcheck clean distclean check-secrets check-docker
+.PHONY: all iso live live-qemu test test-disk test-iso qemu shellcheck clean distclean check-secrets check-docker
 
 all: iso
 
@@ -32,6 +32,11 @@ iso: check-secrets check-docker
 live: check-secrets check-docker
 	$(PROJECT_DIR)/iso/build-live-iso.sh
 
+# Launch the live ISO in QEMU with 12 GB RAM so nix packages fit in the tmpfs.
+# The live session is a pure-RAM tmpfs overlay; Cinnamon + nix need ~7-8 GB total.
+live-qemu:
+	$(PROJECT_DIR)/tests/launch-live-qemu.sh
+
 test-iso: check-secrets check-docker
 	REBUILD_ISO=1 $(PROJECT_DIR)/tests/run-qemu-test.sh